1. Field of the Invention
Embodiments of the present invention relate to a droplet discharge detection device that detects a droplet discharging state of a head array unit and an image forming apparatus including the droplet discharge detection device.
2. Description of the Related Art
An inkjet recording apparatus has been known as an image forming apparatus such as a compound machine where a printer, a facsimile device, a copier, and a plotter are arbitrary combined. For example, the inkjet recording apparatus has been known as the image forming apparatus for which a droplet discharge recording method has been adopted. In the droplet discharge recording method, a recording head is utilized. The recording head discharges ink droplets. The image forming apparatus, for which the droplet discharge recording method has been adopted, discharges the ink droplets onto a sheet of paper being conveyed, and thereby forms an image (recording, typing, seal copying, and printing are used as synonyms).
In such an image forming apparatus (hereinafter, it will be referred to as the “inkjet recording apparatus”), the recording head performs recording by discharging the ink from nozzles onto the sheet of paper. Therefore, image quality degrades when a discharging failure occurs due to, for example, an increase of the ink viscosity caused by evaporation of a solvent from the nozzles, solidification of the ink, adhesion of dust, or mixing in bubbles.
A droplet discharge detection device has conventionally been known such that it detects a droplet discharging state of a recording head. For example, a droplet discharge detection device has been known where a direct light method has been adopted for detecting presence or absence of discharging of a droplet (cf. Patent Document 1 (Japanese Patent Laid-Open Application No. 2007-118264)). In the droplet discharge detection device, a light emitter is arranged at one side of a nozzle array of a recording device so as to emit laser light along a nozzle array, and a light receiver is arranged at the other side of the nozzle array so as to receive the light. Additionally, another droplet discharge detection device has been known where a forward-scattered light method has been adopted for detecting the presence or absence of the discharging of the droplet (cf. Patent Documents 2-6 (Japanese Patent Laid-Open Applications No.2008-12782, No. 2009-113305, No. 2009-132025, No. 2011-31532, and No. 2011-83965)). In the droplet discharge detection device, laser light is emitted from one side of a nozzle array of a recording head along the nozzle array. A light receiver is disposed at a position separated from an optical axis of the laser light at the other side of the nozzle array. The light receiver receives the light that has been scattered by the droplet.
When the droplet discharging state is detected by the forward-scattered light method, the light receiver is disposed at the position that is separated from the optical axis of the laser light so as to prevent the light receiver from receiving the direct light. That is, if the light receiver is disposed at a position close to the optical axis of the laser light, the light receiver may receive the direct light. A droplet discharge detection device has been known in which a light capture unit is disposed for preventing the laser light travelling straight without scattered by the droplet from straying to enter the light receiver disposed at the position separated from the optical axis (cf. Patent Document 2 (Japanese Patent Laid-Open Application No. 2008-12782) and Patent Document 3 (Japanese Patent Laid-Open Application No. 2009-113305)). Additionally, a method has been proposed where flare light of the laser light is prevented or adjusted by shielding (cf. Patent Document 4 (Japanese Patent Laid-Open Application No. 2009-132025), Patent Document 5 (Japanese Patent Laid-Open Application No. 2011-31532), and Patent Document 6 (Japanese Patent Laid-Open Application No. 2011-83965)).
As the flare light, light can be considered such that it has been internally reflected or scattered on a transmitting surface of a lens, at a periphery of a lens, or at a holder portion. However, since such light repeats reflections multiple times or travels along a random light path, the coherence is small. Therefore, such light gives only a small influence on an amount of scattered light, which depends on whether the droplet is discharged or not discharged.
On the other hand, the diffracted light from an aperture of a light emitter that travels in the direction toward the light receiver has higher coherence than that of the internally reflected light or the scattered light. The diffracted light from the aperture of the light emitter interferes with the scattered light from the droplet on the light receiver. Therefore, the diffracted light from the aperture of the light emitter has an effect to vary a light intensity depending on a variation of an optical path difference. The optical path difference may vary depending on an alignment error of the nozzles or on curved discharging. It is possible that the light scattered by the droplet vanishes due to such interference. The problem to be solved is to provide a droplet discharge detection device and an image forming apparatus including the droplet discharge detection device, with which discharging of a droplet can be detected while the light scattered by the droplet is prevented from vanishing due to the interference.